EXHIBIT 5.1

                        [DORSEY & WHITNEY LLP LETTERHEAD]

                                  July 9, 1998

Enzon, Inc.
20 Kingsbridge Road

Piscataway, New Jersey 08854

     Re:  Registration Statement on Form S-3 (File No. 333-58269)
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Ladies and Gentlemen:

          You have  requested  our opinion with respect to the  registration  by
     Enzon,  Inc. (the "Company")  pursuant to a Registration  Statement on Form
     S-3 (the  "Registration  Statement")  under the  Securities Act of 1933, as
     amended (the "Act"),  of an aggregate of 3,983,000 shares (the "Shares") of
     the Company's  Common Stock,  $.01 par value per share (the "Common Stock")
     which  may be sold  from  time to time by the  selling  stockholders  named
     therein (the "Selling Stockholders").  

          In so acting,  we have  examined  originals  or copies,  certified  or
     otherwise  identified to our  satisfaction,  of such  documents,  corporate
     records,  certificates of public  officials and other  instruments and have
     conducted  such  other  investigations  of fact  and law as we have  deemed
     relevant  and  necessary  to form a  basis  for  the  opinions  hereinafter
     expressed.  In conducting  such  examination,  we have assumed (i) that all
     signatures are genuine,  (ii) that all documents and instruments  submitted
     to us as copies conform with the originals, and (iii) the due execution and
     delivery  of  all  documents   where  due  execution  and  delivery  are  a
     prerequisite to the effectiveness thereof. As to any facts material to this
     opinion, we have relied upon statements and representations of officers and
     other  representatives  of the Company and certificates of public officials
     and have not independently verified such facts.

          Based  solely upon the  foregoing,  it is our opinion that the Shares,
     when  issued in  accordance  with the terms of the  Common  Stock  Purchase
     Agreement  dated as of June  25,  1998 by and  among  the  Company  and the
     Selling  Stockholders,  will  constitute  validly  issued,  fully  paid and
     non-assessable shares of Common Stock of the Company.

          Our  opinions  expressed  above are limited to the law of the State of
     New York, the Delaware General Corporation Law, and the federal laws of the
     United States of America.



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          We hereby  consent to the filing of this  opinion as an exhibit to the
     Registration Statement,  and to the reference to our firm under the heading
     "Legal  Matters" in the Prospectus  constituting  part of the  Registration
     Statement relating to the registration of the Shares.

                                       Very truly yours,

                                       

                                       /S/ DORSEY & WHITNEY LLP
                                       ------------------------
                                        Dorsey & Whitney LLP





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